Switching power supplies are well known. Such power supplies are described, for example, in U.S. Pat. No. 4,037,271 entitled "Switching Regulator Power Supply" and No. 4,061,931 entitled "Switching Regulator Power Supply Main Switching Transistor Turn Off, Speed Up Circuit" issued on inventions assigned to Boschert, Incorporated,the assignee of this application. In a switching power supply, the duty cycle of the internal wave form is determined by a feedback signal arranged so as to control the average (rectified) output voltage at a substantially constant level despite variations in line voltage and load current. Typically, prior art switching power supplies have used numerous active semiconductor components in the circuitry controlling this output voltage. While some switching power supplies have not used such components, those power supplies which do use such components are relatively expensive and sometimes exhibit sensitivity to noise. Indeed sometimes these power supplies are damaged by aberrational signals. Accordingly, it would be desirable to have a switching power supply which avoids even these rare instances of failure and at the same time is significantly less expensive to build.